The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device operable in a 1.3 .mu.m or 1.5 .mu.m wavelength band.
Today, a telecommunication trunk generally uses an optical telecommunication system in which optical fibers carry information traffic in the form of optical signals. Currently, quartz glass optical fibers having an optical transmission band of 1.3 .mu.m or 1.5 .mu.m wavelength are used commonly. In correspondence to the foregoing specific transmission band of the optical fibers, current optical telecommunication systems generally use a GaInAsP double-heterojunction laser diode that includes an active layer of In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y and a cladding layer of InP. In such a GaInAsP double-heterojunction laser diode, the carriers are accumulated in the active layer by a potential barrier formed in the conduction band and the valence band between the GaInAsP active layer and the InP cladding layer, and stimulated emission of photons is substantially facilitated in the active layer by the carriers thus accumulated therein. In order to obtain a laser oscillation at the wavelength that matches the optical transmission band of the quartz glass optical fibers, the compositional parameter x for Ga and the compositional parameter y for As are adjusted appropriately.
However, such a conventional laser diode that uses a double-heterojunction of GaInAsP and InP has suffered from the problem of relatively large threshold current of laser oscillation and poor temperature characteristic, primarily due to the relatively small band discontinuity (.DELTA.Ec) of the conduction band between the GaInAsP active layer and the InP cladding layer. More specifically, the electrons escape easily from the active layer in such an GaInAsP laser diode because of the small potential barrier .DELTA.Ec formed by the foregoing band discontinuity, and a large drive current has to be supplied in order to sustain a laser oscillation in the active layer. This problem becomes particularly acute at high temperatures in which the carriers experience an increased degree of thermal excitation. Further, the foregoing GaInAsP laser diode has a problem in that the laser oscillation wavelength tends to shift to a longer wavelength side at high temperatures due to the temperature dependence of the bandgap of GaInAsP. It should be noted that the bandgap of GaInAsP decreases with temperature. This shift of the laser oscillation wavelength raises a serious problem particularly in a wavelength multiplex transmission process of optical signals.
In order to avoid the foregoing problems, conventional GaInAsP double-heterojunction laser diodes for use in optical telecommunication trunk or submarine optical cable systems have used a temperature regulation device, such as a Peltier cooling device, such that the operational temperature of the laser diode is maintained at a predetermined temperature.
On the other hand, there is a strong impetus to expand the use of optical telecommunication technology from the telecommunication trunks to subscriber systems or home systems. In relation to this, there is a demand for an optical semiconductor device suitable for use in home terminals.
When realizing such optical home terminals, it is essential that the optical home terminal is compact and low cost. Further, the optical home terminal should consume little electric power. In order to meet such demands, it is necessary to provide a laser diode that is operable in the 1.3 or 1.5 .mu.m band with a low threshold current and simultaneously without a temperature regulation.
As long as the foregoing GaInAsP/InP double-heterojunction system is used, the foregoing demand cannot be satisfied. Thus, efforts are being made to construct a laser diode having an active layer of GaInAs on a GaAs substrate such that a large band discontinuity .DELTA.Ec is secured in the conduction band. By increasing the In content in the GaInAs active layer, it is possible to reduce the bandgap energy Eg of the active layer, and the oscillation wavelength of the laser diode approaches the desired 1.3 .mu.m band. However, such an increase of the oscillation wavelength by increasing the In content in the GaInAs active layer is successful only to the point in which the oscillation wavelength reaches about 1.1 .mu.m. Beyond that, the lattice misfit between the GaInAs active layer and the GaAs substrate becomes excessive and the epitaxial growth of the GaInAs active layer on the GaAs substrate is no longer possible. It should be noted that the foregoing limit of 1.1 .mu.m takes into consideration the contribution of compressive strain that acts in the direction to increase the oscillation wavelength of the laser diode.
In view of the foregoing situation, Japanese Laid-Open Patent Publication 7-193327 proposes a laser diode operable in the 1.3 or 1.5 .mu.m band, in which an active layer of GaInAs is sandwiched by a pair of cladding layers having a composition set such that a large band discontinuity .DELTA.Ec is secured between the active layer and the cladding layer and that the cladding layer has simultaneously a lattice constant close to that of a strained buffer layer provided on a GaAs substrate with a composition of Ga.sub.0.8 In.sub.0.2 As. However, the proposed device is deemed to be unrealistic in view of the large lattice misfit between the active layer and the GaAs substrate. It is believed that the existence of such a large lattice misfit reduces the lifetime of the laser diode substantially.
On the other hand, Japanese Laid-Open Patent Publication 6-37355 describes a compound semiconductor structure that includes a GaInNAs mixed crystal film formed on a GaAs substrate. By adding N to GaInAs, it becomes possible to form the GaInNAs film with a lattice constant that matches the lattice constant of GaAs. The GaInNAs film thus added with N has a reduced bandgap due to a large negative bowing of the bandgap-composition relationship observed in a GaAs--GaN system. Thus, it is expected that a double-heterostructure laser diode having an oscillation wavelength in the 1.3 or 1.5 .mu.m and simultaneously a large band discontinuity .DELTA.Ec necessary for carrier accumulation, may be obtained by using GaInNAs for the active layer. As the GaInNAs film can have a composition that establishes a lattice matching with GaAs, it is possible to use an AlGaAs or GaAs cladding in combination with the active layer of GaInNAs.
FIG.1 shows the compositional change of a bandgap Eg for a GaAs-GaN system according to the Japanese Laid-open Patent Publication 6-37355.
Referring to FIG. 1, it will be noted that the endmember component GaN has a very large bandgap Eg of about 3.5 eV, contrary to the endmember component GaAs, of which bandgap Eg is only about 1.4 eV. Thus, GaN is expected to be one of the most promising materials of an active layer for an optical semiconductor device that is operable in a blue or ultraviolet wavelength band.
The striking feature of FIG. 1 is that the compositional change of the bandgap Eg between GaAs and GaN is not linear but there appears a very significant negative bowing. Probably, this large negative bowing of bandgap is related to the existence of a very large difference in the atomic radius between As and N. In fact, there is reported a large miscibility gap in the GaAs--GaN system.
Thus, a bandgap Eg as small as about 1.2 eV is possible for a GaNAs system by incorporating N into a GaAs crystal with a proportion of about 10 mole %. While the GaNAs system of this composition has a small lattice constant due to the small atomic radius of N, a satisfactory lattice matching can be achieved, with respect to a GaAs substrate, by incorporating In.
FIG. 2 shows the construction of a laser diode 1 proposed in the Japanese Laid-Open Patent Publication 7-154023, op cit.
Referring to FIG. 2, the laser diodel1 is constructed on a substrate 10 of n-type GaAs and includes a lower cladding layer 11 of n-type GaInP provided on the GaAs substrate 10. On the lower cladding layer 11, an active layer 12 of undoped GaInNAs is provided, and an upper cladding layer 13 of p-type GaInP is provided further on the active layer 12, wherein the upper cladding layer 13 is formed with a ridge structure extending in an axial direction of the laser diode. Further, a pair of current confinement structures 14 of n-type GaAs are provided at both lateral sides of the ridge structure, and a contact layer 15 of p-type GaAs is provided on the ridge structure so as to bury the current confinement structures 14 underneath. Further, a p-type ohmic electrode 16 is provided on the contact layer 15, and an n-type ohmic electrode 17 is provided on the bottom surface of the substrate 10.
In operation, holes are injected into the active layer 12 from the electrode 16 via the contact layer 15 and the ridge structure of the upper cladding layer 13, wherein the active layer 12 is further injected with electrons from the electrode 17 via the substrate 10 and the lower cladding layer. Thereby, a stimulated emission of photons occur in the active layer 12 as a result of recombination of the electrons and holes thus accumulated in the active layer 12, and the laser diode oscillates at the wavelength of 1.3 .mu.m or 1.5 .mu.m corresponding to the characteristically increased bandgap of the active layer 12.
In the laser diode of FIG. 2, the current confinement structures 14 restricts the current path of the holes by establishing a p-n junction between the current confinement structure 14 and the contact layer 15, wherein it should be noted that each current confinement structure 14 is formed above the active layer 12, with a part of the upper cladding layer 13 intervening between the current confinement structure 14 and the active layer 12.
While it is usual in conventional laser diodes to form a current confinement structure corresponding to the current confinement structure 14, such that the current confinement structure reaches the lower cladding layer 11 or to the substrate 11, across the active layer 12, such a construction, when applied to the active layer 12 having the composition of GaInNAs, would cause a problem of extensive defect formation at the lateral edges of the active layer 12 where the active layer 12 is laterally bounded by the GaAs current confinement structures 14. It should be noted that such a formation of the current confinement structure includes a mesa formation step for forming the ridge structure, while the etching process used in such a mesa formation step tends to introduce a substantial amount of defects into the semiconductor layer thus is subjected to the etching process. The active layer 12 containing N therein is particularly susceptible to defects, in view of rapid deteriorating crystal quality with increasing content of N in the crystal.
In the device of FIG. 2, the problem of defect formation and associated non-optical recombination of carriers is successfully avoided by forming the current confinement structures 14 above the active layer 12 with a separation therefrom. On the other hand, such a construction naturally allows a lateral diffusion of the injected holes away from the region immediately under the ridge structure as indicated by arrows in FIG. 2. Thereby, the threshold current of laser oscillation increases and the efficiency of laser oscillation is deteriorated substantially.